Newton's Rings

A plano-convex lens having a radius of curvature of r = 4.00m is placed on a concave glass surface whose radius of curvature is R = 12.0 m. Determine the radius of the 100th bright ring, assuming 500-nm light is incident normal to the flat surface of the lens.

2. Relevant equations

Kinda need to derive your own here. What I came up with was r - sqrt(r2 - s2) - R + sqrt(R2 - s2) = (m+.5)lambda/(2nfilm)

s is the radius of the mth bright ring, lambda is the wavelength of light, nfilm is 1 for this problem since I assume the film is air